Add functional form converters

[emarinri]

This PR adds the necessary functions to perform functional form conversions. It has the following features

• Functions to convert coefficients between functional forms. These are included in bond_converters.py, angle_converters.py, dihedral_converters.py
• A wrapper function that dispatches calls to each of the converters.
• The wrapper function uses a registry of converter functions that is built via decorating each of the converters.
• Tests for the converters
• Modified the metadata to include a canonical representation of a functional form (in similar fashion of what we currently have with the LJ-AB potential)